Ch. 10: Name Control

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1 Ch. 10: Name Control

2 Static elements from C The static keyword was overloaded in C before people knew what the term overload meant, and C++ has added yet another meaning. The underlying concept with all uses of static seems to be something that holds its position (like static electricity), whether that means a physical location in memory or visibility within a file.

3 Static elements from C In both C and C++ the keyword static has two basic meanings: Allocated once at a fixed address; that is, the object is created in a special static data area (a.k.a. static storage) rather than on the stack each time a function is called. See next slide Local to a particular translation unit (and local to a class scope in C++, as you will see later). Here, static controls the visibility (a.k.a. scope) of a name, so that name cannot be seen outside the translation unit or class. This also describes the concept of linkage, which determines what names the linker will see.

4 Static storage Contains: Local auto objects Contains: Objects allocated dynamically Originally it meant Block Started by Symbol Contains: Local, static and constant objects Unitialized global objects Static storage Remember: In C (and C++) uninitialized static variables are initialized by default with 0. Source: Leo Ferres, Memory management in C: The heap and the stack

5 Static elements from C Review the next part of this section until Other storage class specifiers

6 Namespaces In a large project, lack of control over the global name space can cause problems. To solve these problems for classes, vendors often create long complicated names that are unlikely to clash, but then you re stuck typing those names. It s not an elegant, language-supported solution.

7 Namespaces You can subdivide the global name space into more manageable pieces using the namespace feature of C++. The namespace keyword, similar to class, struct, enum, and union, puts the names of its members in a distinct space.

8 Namespaces A namespace definition can appear only at global scope, or nested within another namespace. No terminating semicolon is necessary after the closing brace of a namespace definition. A namespace definition can be continued over multiple header files using a syntax that, for a class, would appear to be a redefinition:

10 A namespace name can be aliased to another name, so you don t have to type an unwieldy name created by a library vendor:

11 Static members in C++ Sometimes we need a single storage space to be used by all objects of a class. We could use a global variable, but this is not very safe: Global data can be modified by anyone Its name can clash with other identical names in a large project. It would be ideal if the data could be stored as if it were global, but be hidden inside a class, and clearly associated with that class. example on next slide

12 The initialization of a static member must be done outside of the class declaration, in the definitions section (file)! Important: Do not confuse with the static constants from Ch.8!

13 Actually, even the static integer constants from Ch.8 can be defined outside the class (It is consistent with ODR!):

14 QUIZ Define a class Turtle with a static data member named pop (population), initialized with zero. The constructor increments pop.

16 Static arrays (const or not) With static consts of integral types you can provide the definitions inside the class, but for everything else (including arrays of integral types, even if they are static const) you must provide a single external definition. example on next slide

18 Note that the static keyword is not repeated at initialization!

19 Note: C++11 has introduced the related qualifier constexpr, which allows more definitions inside the class:

20 SKIP Nested and local classes

21 Static member functions static member functions, like static data members, work for the class as a whole rather than for a particular object of a class. Instead of making a global function that lives in and pollutes the global or local namespace, you bring the function inside the class.

22 Static member functions You can call a static member function in the ordinary way, with the dot or the arrow, in association with an object. However, it s more typical to call it by itself, without any specific object, using the scope-resolution operator:

23 Static member functions Cannot access ordinary data members, only static data members Can call only other static member functions Normally, the address of the current object (this) is quietly passed in when any member function is called but a static member has no this (which is the reason it cannot access ordinary members) Thus, we get the tiny increase in speed afforded by a global function because a static member function doesn t have the extra overhead of passing this. At the same time you get the benefits of having the function inside the class.

24 Static member function example

25 QUIZ: Is this code correct?

26 Solution: Yes. Although the function call in the main program looks weird, it is legal for a static member function!

27 SKIP Static initialization dependency Alternate linkage specifications No homework assigned for this chapter! Individual work (preparation for midterm): End-of-chapter 2, 3, 4, 16

QUIZ. What is wrong with this code that uses default arguments?

QUIZ. What is wrong with this code that uses default arguments? QUIZ What is wrong with this code that uses default arguments? Solution The value of the default argument should be placed in either declaration or definition, not both! QUIZ What is wrong with this code